1. Field of the Invention
The present invention is directed to heterojunction bipolar transistors (HBTs). More specifically, the present invention relates to HBTs for use as high power microwave amplifiers in S-X bands (3-12 GHz).
2. Description of the Related Art
The HBT concept is well known and has been successfully applied to several material systems e.g. Al.sub.x GA.sub.1-x As/GaAs,Si.sub.1-x Ge.sub.x etc. Generally, the HBTs have been made on GaAs or Si substrates and suffer from three primary material limitations: (1) low electric breakdown field, (2) low saturated drift velocity of electrons, and (3) low thermal conductivity. All of these factors limit the power output of the device at a given frequency and the maximum frequency of operation.
Currently, silicon bipolar devices are being used in the UHF to S-Band frequency range while Al.sub.x GA.sub.1-x As/GaAs HBTs are typically used at X-Band. Si.sub.1-x Ge.sub.x HBTs are expected to outperform silicon bipolar transistors at S-band, with the possibility of operation to X-Band.
In the X-band, the Al.sub.x GA.sub.1-x As/GaAs HBTs produce 3-4 W/mm of RF power at 10 GHz and at room temperature. In addition, both Si and GaAs HBTs are capable of working at a junction temperature of 150.degree. C.
U.S. Pat. No. 4,985,742 to Pankove discloses a transistor in which an n-type gallium nitride layer and a p-type silicon carbide layer from a heterojunction. The n-type gallium nitride layer serves as the emitter, the p-type silicon carbide layer serves as the base and a n-type silicon carbide layer, formed beneath the p-type silicon carbide layer serves as the collector. As disclosed in the related articles, "New Transistors take the Heat", Machine Design, Aug. 10, 1995 p. 36 and Pankove et al., "High-Temperature GaN/SiC Heterojunction Bipolar Transistor with High Gain", December 1994 IEEE, pp. 15.6.1-15.6.4, such a device can operate at high temperatures (up to 500.degree. C.) and have high current gains (greater than 10.sup.5 at room temperature and around 100 at 500.degree. C.)
However, the need still exists for high power transistors for use in the microwave region, especially one having improved frequency response at around 3-12 GHz. The HBT structure disclosed by Pankove does not address microwave applications and is not designed for operation at high frequencies.